Pour the vegetable broth into a medium saucepan and place over low heat, keeping it gently steaming but not boiling throughout the entire cooking process. Warm broth is essential — cold broth will disrupt the cooking and affect the final texture of the risotto.
Blanch 1 cup of the peas in salted boiling water for 1–2 minutes, then immediately transfer to a bowl of ice water to preserve the bright green color. Drain, blend with 2–3 tablespoons of warm broth and a pinch of salt until smooth, then set aside.
Heat the olive oil and 1 tablespoon of butter in a heavy-bottomed pan over medium heat, then add the diced onion with a pinch of salt and cook for 5–7 minutes until very soft and translucent. Add the minced garlic and cook for 1 more minute, stirring constantly, until fragrant but not browned.
Add the Arborio rice to the pan and stir to coat every grain in the oil and butter. Toast for 1–2 minutes, stirring continuously, until the edges of the grains look slightly translucent and the rice smells faintly nutty.
Pour in the white wine all at once and stir constantly until it has been fully absorbed by the rice, about 2–3 minutes. The pan should look almost dry before you proceed to adding the broth.
Add the warm broth one ladle (about ½ cup) at a time, stirring frequently and waiting until each addition is nearly absorbed before adding the next. Continue for 20–25 minutes until the rice is al dente — tender with a slight bite in the center.
About 3 minutes before the risotto is done, stir the remaining 1 cup of whole peas directly into the pan. They will heat through and cook gently in the last few minutes, adding pops of sweetness and texture throughout.
Remove the pan from the heat, then stir in the remaining 2 tablespoons of cold butter, the grated Parmesan, the pea puree, lemon zest, lemon juice, and torn mint leaves until fully combined and glossy. Taste and season generously with salt and black pepper.
If the risotto is too thick, stir in a splash of warm broth until it flows like a wave when you tilt the pan. Divide immediately between warm bowls and garnish with extra Parmesan, fresh mint leaves, a pinch of lemon zest, a drizzle of olive oil, and cracked black pepper.